How to parse CSV file in lightning Javascript

There are many good sample provided which you can use and refer. For eg:

/**
 * Used to read a delimited file.
 */
public class SSSCsvReader {
    private String delim = ',';
    // the input data
    private String[] buffer;

    public SSSCsvReader(String data){
        this.buffer = data.split('\n'); 
    }

    public SSSCsvReader(String data, String delim){
        this.buffer = data.split('\n'); 
        this.delim = delim;
    }

    /**
     * Read and parse next available line.  Return null if end of stream.
     */
    public String[] readLine(){
        if(buffer.size() == 0)
            return null;
        String line = this.buffer.remove(0);
        String[] parts = new String[] {};
        while(line != ''){
            Integer next = 0;
            if(line.startsWith('"')){
                line = line.substring(1); // strip initial
                Integer quoteIndex = findQuote(line, 0);
                while(quoteIndex == -1){
                    if(buffer.size() == 0){
                        // EOT!
                        quoteIndex = line.length();
                    } else { 
                        // grab the next line
                        Integer skip = line.length();
                        line += '\n' + this.buffer.remove(0);
                        quoteIndex = findQuote(line, skip);
                    }
                }
                // advance to comma
                next = quoteIndex + 1;
                parts.add(line.substring(0, quoteIndex).replace('""', '"'));
            } else {
                next = line.indexOf(this.delim, next);
                if(next == -1)
                    next = line.length();
                // NB in Substring, "endindex" is the index of the character AFTER the last index to get
                parts.add(line.substring(0, next));
            }       
            if(next == line.length() - 1)
                // case of a terminating comma.
                parts.add('');
            line = next < line.length() ? line.substring(next+1) : '';
        }
        if(parts.size() == 0)
            // empty string - we still want to return something...
            parts.add('');
        return parts;
    }

    static private Pattern quotePattern = Pattern.compile('(?<!")"(?!")');
    /**
     * Find next quote in the line
     */
    private Integer findQuote(String line, Integer skip){
        Matcher m = quotePattern.matcher(line);
        m.region(skip, m.regionEnd());
        if(!m.find())
            return -1;
        return m.start();
    }
}

It follow approach that if your string contains comma it will be in double quote and then it parse that.

SalesForce Apex CSV Parser